Nociceptors are primary sensory afferent (C and Aδ fibers) neurons that are activated by a wide variety of noxious stimuli including chemical, mechanical, thermal, and proton (pH <6) modalities. The lipophillic vanilloid, capsaicin, activates primary sensory fibers via a specific cell surface capsaicin receptor, cloned as the transient receptor potential vanilloid-1 (TRPV1). The intradermal administration of capsaicin is characterized by an initial burning or hot sensation followed by a prolonged period of analgesia. The analgesic component of TRPV1 receptor activation is thought to be mediated by a capsaicin-induced desensitization of the primary sensory afferent terminal. Thus, the long lasting anti-nociceptive effect of capsaicin has prompted the clinical use of capsaicin analogs as analgesic agents. Further, capsazepine, a capsaicin receptor antagonist, can reduce inflammation-induced hyperalgesia in animal models. TRPV1 receptors are also localized on sensory afferents which innervate the bladder. Capsaicin or resiniferatoxin has been shown to ameliorate incontinence symptoms upon injection into the bladder.
The TRPV1 receptor has been called a “polymodal detector” of noxious stimuli since it can be activated in several ways. The receptor channel is activated by capsaicin and other vanilloids and thus is classified as a ligand-gated ion channel. TRPV1 receptor activation by capsaicin can be blocked by the competitive TRPV1 receptor antagonist, capsazepine. The channel can also be activated by protons. Under mildly acidic conditions (pH 6-7), the affinity of capsaicin for the receptor is increased, whereas at pH<6, direct activation of the channel occurs. In addition, when membrane temperature reaches 43° C., the channel is opened. Thus heat can directly gate the channel in the absence of ligand. The capsaicin analog, capsazepine, which is a competitive antagonist of capsaicin, blocks activation of the channel in response to capsaicin, acid, or heat.
The channel is a nonspecific cation conductor. Both extracellular sodium and calcium enter through the channel pore, resulting in cell membrane depolarization. This depolarization increases neuronal excitability, leading to action potential firing and transmission of a noxious nerve impulse to the spinal cord. In addition, depolarization of the peripheral terminal can lead to release of inflammatory peptides such as, but not limited to, substance P and CGRP, leading to enhanced peripheral sensitization of tissue.
Recently, two groups have reported the generation of a “knock-out” mouse lacking the TRPV1 receptor. Electrophysiological studies of sensory neurons (dorsal root ganglia) from these animals revealed a marked absence of responses evoked by noxious stimuli including capsaicin, heat, and reduced pH. These animals did not display any overt signs of behavioral impairment and showed no differences in responses to acute non-noxious thermal and mechanical stimulation relative to wild-type mice. The TRPV1 (−/−) mice also did not show reduced sensitivity to nerve injury-induced mechanical or thermal nociception. However, the TRPV1 knock-out mice were insensitive to the noxious effects of intradermal capsaicin, exposure to intense heat (50-55° C.), and failed to develop thermal hyperalgesia following the intradermal administration of carrageenan.
Further, as known in the art, cytochrome P450 enzymes are heme-containing membrane proteins localized in the smooth endoplasmic reticulum of numerous tissues, including, in particular, the liver. This family of enzymes catalyzes a wide variety of oxidative and reductive reactions and has activity towards a chemically diverse group of substrates. These enzymes are the major catalysts of drug biotransformation reactions and also serve an important detoxification role in the body. CYP3A is both the most abundant and most clinically significant subfamily of cytochrome P450 enzymes. The CYP3A subfamily has four human isoforms, 3A4, 3A5, 3A7 and 3A43, with CYP3A4 being the most commonly associated with drug interactions. The CYP3A isoforms make up approximately 50% of the liver's total cytochrome P450 and are widely expressed throughout the gastrointestinal tract, kidneys and lungs and therefore are ultimately responsible for the majority of first-pass metabolism of drugs and toxins, leading to their disposition from the body. This is important as increases or decreases in first-pass metabolism can have the effect of administering a much smaller or larger dose of drug than usual. Inhibition of these enzymes can possibly lead to life-threatening conditions where the enzyme is not able to perform this function. More than 150 drugs are known substrates of CYP3A4, including many of the opiate analgesics, steroids, antiarrhythmic agents, tricyclic antidepressants, calcium-channel blockers and macrolide antibiotics.
Certain VR1 antagonists are discussed in U.S. Pat. No. 6,933,311. However, interactions with CYP3A4 for these compounds are unknown.
Accordingly, the need exists to develop TRPV1 antagonists that exhibit low inhibitory activity against drug metabolism enzyme CYP3A4. Such antagonists should possess a much lower risk of drug-drug interactions with CYP3A4 in human drug metabolism and serve well as safe pharmaceutical agents with fewer limitations on therapies involving combinations of drugs. Such compounds should also provide beneficial pharmaceutical characteristics while minimizing undesirable side effects generally associated with inhibition of cytochrome P450 enzymes.